In particular, this invention is applicable to the industrial control of highly absorbant objects with large dimensions and for which it is desired to restore the images of plane cuttings by using a high-energy (for example, greater than several 100 KeV) pulsed X-ray source and means for processing the signals derived from radiation detectors having traversed this object. This invention may also be applied to the control of slightly absorbent objects with small dimensions.
There exists a known type of X-ray tomography device which makes it possible to obtain the image of at least one plane cutting of an object and includes an X-ray source supplying high-energy pulses at a predetermined frequency. This device also includes a primary collimator so that the pulses traverse the object inside a cutting plane and close to the latter. Means are usually provided to support the object and to move it in translation and/or in rotation. This type of device also includes a small bar of measuring detectors receiving the attenuated energy pulses having traversed the object and a secondary collimator placed between the object and the bar of detectors. These detectors respectively supply on outputs electric detection pulses representing the received attenuated energy pulses derived from the object. Finally, this known type of device includes display and processing means connected to the outputs of the detectors in order to supply the image of each cutting of the object. The signals derived from the detectors are generally coded and digitalized so as to enable them to be processed by a computer. In this type of device, the detectors may be constituted by photoelectron multipliers or photodiodes respectively connected to a scintillator.
In this known type of device, the pulsed X-ray source does not present perfect stability and the detection means may present temperature drifts and dark currents and, in the case of a radioactive object, there exists an unwanted detection not taken into account when processing signals. Accordingly, the image of each cutting obtained is not perfect.
When the detection means used are constituted by photoelectron multipliers or photodiodes connected to scintillators, these scintillators age under the effect of radiation thus, the transparency and efficiency of the scintillators become altered after absorbing a certain dose of X-rays. It is then necessary to regenerate the scintillators via a thermal cycle.
Owing to the foregoing, it frequently happens that any tomography device which uses scintillators is not available. Moreover, the volume occupied by a set of scintillators and photodetectors (photoelectron multipliers or photodiodes) limits the possibilities of embodying detection means with a small geometric pitch, which strictly limits the number of measuring points. Finally, the limited dynamics of photoelectron multipliers involves the use of photodiodes for objects with high absorption dynamics, but in this case necessary to take into account two energy conversion efficiencies: the efficiency relating to the conversion of X photons into visible photos and the efficiency relating to the conversion of visible photons into electrical charges. As a result, there is extremely high loss of sensitivity of the devices thus constituted allied with the introduction of significant statistical noise. In general, the scintillation efficiency is extremely low (less than 10% ).
Generally speaking, it is therefore preferable to use detection means which make it possible to carry out a direct conversion of the X photons into electrical charges. These detection means usually consist of small bars of detectors containing a semiconductor.
These detectors are used in a known way with low energy continuous radiation sources (normally with X-ray generators fed with a d.c. voltage of about several 10 k volts). These detectors are distributed in the form of a small bar (or a mosaic), each detector being constituted by a semiconductive pellet of small dimensions (about a few mm) inserted between two electrodes, the radiation to be detected arriving under normal incidence at the planes of the detectors and the electrodes.
This geometry of the detectors does not enable said detectors to be used with good efficiency for high energies. In fact, with such a structure, the increase of the thickness of the semiconductive pellet, rendered necessary so as to obtain high energy effectiveness, introduces a large collection distance of the charges created. This introduces two major drawbacks:
the charges can recombine before reaching an electrode,
the maximum charges collection time is considerable.
Owing to this, the known tomography devices functioning with high-energy X-ray sources do not use detectors containing a semiconductor.
In known pulsed source tomography devices, the signals supplied respectively by the detectors are pulse signals which present extremely high drag when the flow of X radiations received has extremely high energy. As a result, the pulse signal supplied by a detector after receiving an X-ray pulse having traversed the object may be superimposed on the pulse signal supplied by this detector at the time of receiving the X-ray pulse received previously. Moreover, these detectors, when being irradiated by the pulses originating from the object, present a relatively high temperature drift.
These known tomography devices functioning with high energy X-ray sources process the signals supplied by the detectors without taking account of the drag of the electrical pulse signal supplied by each detector, any temperature drifts provoked by the irradiation of these detectors and the dark current of each detector. These devices no longer take account of the instability of the X-ray pulsed source and any possible radiation emitted by the object itself (radioactive object). As a result, the image of each section of the object is an image of poor quality.